In the following discussion, the leading digits of a reference numeral indicate the first figure in which a particular element is presented. The invention relates generally to interferometry and more particularly to dilatometry. A dilatometer is utilized to measure the thermal coefficient of expansion of a specimen. In an optical dilatometer, interference effects are utilized to accurately measure the thermal variation of a linear dimension of the specimen.
In the dilatometer shown in FIG. 8 and presented by S. J. Bennet in the article "A Double-Passed Michelson Interferometer", Optics Communications, Vol. 4, Number 6 (Feb/Mar 1972), an incident light beam 81 is split by an input beam splitter 82 into a first light beam 83 and a second light beam 84. A polarizing beam splitter 85, an optical cube-corner 86 and a quarter wave plate 87 direct the first light beam twice from the surface of a platen 88 to which the specimen 89 has been attached. The optical paths involved in each reflection are parallel to each other and are located symmetrically with respect to a line normal to the surface. The second light beam is twice reflected in a similar manner from the surface of the specimen. The first and second light beams are combined by an output beam splitter 810 to produce an interference pattern in an output beam 811. As the temperature of the specimen is varied, the resulting change in the difference between the path lengths of the first light beam and the second light beam is proportional to the linear coefficient of thermal expansion of the specimen. The change in the interference resulting from the change in this path difference is detected to determine this coefficient. Since the distances involved are determined by detecting light that has been twice reflected from the surfaces of the specimen and the platen in the symmetrical manner described, the device is insensitive to translation or rotation. Only the linear expansion of the test object in the direction normal to the surface is detected so that the linear coefficient of expansion may be determined.
Unfortunately, in Bennet's device, the input and output beam splitters are glass parallelpipeds that are traversed by beam 84 but not by beam 83. As a result of this difference between the paths of beams 83 and 84, this device is sensitive to alignment between the input and output beam splitters and is also sensitive to thermal effects on these beam splitters such as thermal expansion and thermal variation of the index of refraction. Since the operation of the dilatometer requires heating or cooling of the specimen, often to extreme temperatures, it is desirable that the optical portion of the device be as insensitive to temperature as possible. For accurate fringe detection, the wavefronts of the emergent light beams must be substantially parallel or else undesired interference across the cross-section of the combined output light beams will result which degrades detection of the desired interference between the combined output light beams. The requirement that the output light beams be parallel requires that the input and output beam splitters be aligned within very strict tolerances (on the order of 10 arcseconds as suggested in the Bennet article). To manufacture and maintain these tolerances is both difficult and expensive.
A dilatometer that eliminates this stringent alignment problem is shown in FIG. 9 and is presented in U.S. Pat. No. 3,788,746 entitled OPTICAL DILATOMETER issued to Richard R. Baldwin and Bruce J. Ruff on Jan. 29, 1974. In that dilatometer, an input beam 91 is divided by an input beam splitter 92 and a beam bender 93 into a first input beam 94 and a second input beam 95. Each input beam has two perpendicularly polarized components that are separately directed by a polarizing beam splitter 96 and a pair of cube corners 97 and 98. The first component of each of beams 94 and 95 passes through beam splitter 96 to cube corner 98 and back out to a pair of detectors 912 and 913, respectively. The second component of each of beams 94 and 95 is directed by polarizing beam splitter 96, cube corner 97 and quarter wave plate 99 to reflect twice off of specimen 911 and platen 910, respectively, in a symmetrical manner as in the article by Bennet and then to pass on into detectors 912 and 913, respectively. Each of these second components interferes with its associated first component. Because neither of the two first components impinges on the platen or the specimen, each of these first components serves to produce a reference path length against which the path length of its associated second component is compared. The outputs of the detectors are compared to determine the linear coefficient of thermal expansion for the specimen.
Although this dilatometer does not require that the input beam splitter 92 by precisely aligned with another beam splitter, as is required to avoid unwanted interference effects in the dilatometer presented by Bennet, there must be a reasonable degree of alignment between input beam splitter 92 and beam bender 93 or else the second component of beam 94 will overlap onto the platen or the second component of beam 95 will overlap onto the specimen. This problem becomes increasingly important with increased distance between the dilatometer and the specimen/platen combination. In addition, the use of two detectors increases the cost and complexity of this dilatometer. Also, the first component of each of beams 94 and 95 reflects off of cube corner 98 but not off of cube corner 97 whereas the second component of each of beams 94 and 95 reflects off of cube corner 97 but not off of cube corner 98. Therefore, cube corner 97 must be precisely aligned with cube corner 98 or else similar undesired interference problems can arise. Misalignment of a cube corner will not affect the direction of either of the components in the output beam, but it can produce a lateral displacement between these two components, thereby affecting the interference effects observed in the output beam. It would be preferable to have a device in which interfering beams passed through all elements of the device in a manner which made the device insensitive to independent small rotations and translations of all of its elements.